Tunable selector of uniform band width



April. 9, 1940. +1.1. WHEELER IUKABLE SELECTOR OFUHIEORI BAND WIDTH F1104 Oct. 29, 1938 FlG.8.

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0 L G R R N m 0 w N F 0 ME 0 6 E W 7 Wm E I ww l v U V D "A E Q R Y. mmm m n mm w OSCILLATOR INVENTOR OLD A. WHEE R ATTORNEY FIGJO.

Patented Apr. 9, 1940 UNITED STATES v 2,196,881 TUNABLE SELECTOR OF UNIFORM BAND WIDTH Harold A. Wheeler, 'Great Neck, N. Y., assignor to Hazeltine Corporation, a corporation of Delaware Application October 29, 1938, Serial No. 237,665

12 Claims.

This invention relates generally to band-pass selectors tunable over a wide range of frequencies with uniform band width and, while the selector of the invention is of general utility, it is of 5 particular utility as a tunable band-pass selector circuit for a modulated-carriertelevision receiver adapted to receive signals from any selected one of a plurality of transmitting stations.

It is sometimes desirable to provide in highfrequency signal-translating apparatus a bandpass selector tunable over a wide range of frequencies and of uniform band width. For instance, such selective circuits are highly desirable in a television receiver adapted to receive sig-.

nals from any selected one of a plurality of transmitting stations operating on different frequencies, since the modulation-band widths of all such transmitting stations are substantially equal. Therefore, it is desirable to provide selecpredetermined band width and which may be tuned to the frequency of any selected transmit ting station. However, tunable selector circuits generally have the undesirable feature that the pass band of the selector varies as the selector is tuned to shift the means frequency of the pass band. In-order to maintain a uniform band width over a wide tuning range, relatively complicated mechanical arrangements have been utilized, in 30 some instances in the prior art, to provide the desired variation of the coupling between the coupled tuned circuits of a selector as the tuning of the circuits is adjusted to vary the mean frequency of the pass band of the selector. Other prior-art selectors have utilized a combination of fixed coupling-reactance elements of both kinds by which any desired variation in the coeflicient of coupling could be approximated over the tuning range. The adjustable reactance elements for tuning the various circuits of selectors of the last-mentioned type have been of the same kind. This type of tunable selector only approximates the desired variation of coupling and uniform bandwidth over the. tuning range.

It is an object of the present invention to provide a band-pass selector which is tunable over a wide. range of frequencies with uniform band width and which is not subject to the abovementioned disadvantages.

It is afurther object of the invention to protive circuits for the receiver which pass only avide a simplified band-pass selector tunable over a wide range of frequencies with uniform band width.

It is still another object of the invention to provide a selector tunable over a wide range of frequencies with uniform band width and comprising a single fixed reactance-coupling element between the tunable circuits of the selector. ,In accordance with the invention a band-pass selector tunable over a wide range of frequencies with uniform band width comprises a capacitance-tunable circuit and an inductance-tunable circuit coupled by a fixed reactance coupling element. The kind of reactance of this fixed reactance element is so chosen with respect to its manner of coupling between the tunable circuits that the resulting coeflicient of coupling between the tunable circuits isinversely proportional to the mean frequency of the pass band of the filter as it is tuned over its range. A uniform band width is obtained by tuning both of the tunable circuits to any given frequency within the tuning range. In accordance with a preferred embodiment of the invention, the fixed-coupling reactance element is a condenser which is included in series in both of the tunable circuits. Also, in the preferred embodiments of the invention, unicontrol means are provided for simultaneously adjusting the tuning elements of the selector so that both tunable circuits are atall times tuned to the same frequency. 1

For a better understanding of the'present invention, together with other and further objects thereof, reference is had to the following description takenin connection with the accompanying drawing, and its scope will be pointe out in the appended claims.

In the accompanying drawing, Fig. lis a circuit diagram of a tunable constantk band-pass filter half-section embodying the invention; Fig. 2 illustrates the band-pass characteristics of the tunable band-pass selectors of the invention; Fig. 3 is a circuit diagram of a form of the invention utilizing a fixed capacitance element as the coupling element thereof; Fig. 4 is a circuit diagram of a form of the invention utilizing a fixed inductance element as the coupling element thereof; Fig. 5 is a diagram of a circuit in which the effect of any incidental inductance in series with the fixed coupling condenser on the band width of a circuit in which the effect of incidental capacitance across the fixed coupling inductance of Fig. 4 on the band width of the selector is compensated; Fig. 7 is a diagram of a circuit in which the efiect of incidental capacitance of the fixed inductance of the circuit of Fig. 3 on the band width is compensated; Fig. 8 is a diagram of a circuit in which the effect of the inherent capacitance of the variable inductance of the circuit of Fig. 3 on the band width is compensated: Fig. 9 is a circuit diagram of a selector of the invention utilized as a selective circuit in a television receiver; while Fig. 10 illustrates a modification of the circuit of Fig. 9.

Referring now more specifically to Fig. 1 of the drawing, there is shown a band-pass selector of the invention which is derived from a constant-k band-pass filter half-section. The half-section comprises a series resonant circuit including inductance L1 and capacitance C1 as a mid-series arm and a parallel-resonant circuit including inductance L: and capacitance C: as a shunt arm. Proper terminating resistances R, R, matching the image impedance of the filter at its terminals, are shown, which resistances may he provided in practice or may be modified in values or comprised in whole or in part in the inherent resistance of associated circuits. Capacitance C1 and inductance La are adjustable. The circuit of Fig. 1 thus comprises a capacitance-tunable circuit including elements R, Li, C1, C2 andan inductance-tunable circuit including elements In, C2. Fixed reactance element C2 is of such type with respect toits manner of coupling. between the tunable circuits as to provide a resultant coefficient of coupling inversely proportional to where, A: is the width of the pass band of the circuit, in angular-frequency, and we is the mean angular frequency of the pass band.

From Equations 1 and 2, it will be seen that the width of the pass band Aw is independent of the mean frequency of the pass band, which is variable only with the values of adjustable capacitance C1 and adjustable inductance In. It is therefore apparent that the circuit of Fig. 1 is tunable over a wide range of frequencies with uniform band width.

It will be understood that the circuit of Fig. 1

- can be utilized with each pair of terminals terminated in resistance equal to the nominal image impedance of the filter or it can be utilized with terminal nesistors of modified values coupled to one or both of the pairs of terminals. Both pairs of terminals must be terminated with proper resistance for critical damping while, if only one pair of terminals is resistance-terminated, there is an advantage that a greater transfer ratio through the filter is procured.

Several half-sections like that of Fig. 1 may be connected in succession if a closer approach to an ideal filter is desired.

In Fig. 2, there are shownthe band pass characteristics of the selector circuit of Fig. l. for.

two different values of mean frequency are, identified as 010 and 0 respectively, indicating that such characteristics are of uniform width and v 2,196,881 of the selector is compensated; Fig. 6 is a diagram similar shape irrespective of the mean frequency to which the selector is tuned.

By an impedance transformation, the circuit of Fig. 3 may be derived from the circuit of Fig. 1. Circuit elements in this and the succeeding figures corresponding to circuit elements of Fig. 1 have been given the same reference characters. The circuit of Fig. 3 includes a capacitance-timable first circuit, including fixed inductance L1 and variable capacitance C1, and an inductancetunable second circuit, including variable inductance L2 and fixed capacitance C2, coupled by a fixed coupling condenser Cc. The following simplified equations apply to the circuit of Fig. 3, if the band-width is much less than the mean frequency of the band:

, where,

k is the coefiicient of coupling between the tunable circuits;

X4: is the reactance of condenser- Cc;

X1 is the reactance of the reactance elements of one type in the capacitance-tunable circuit; and I X: is the reactance of the reactance elements of one type in the inductance-tunable circuit.

Noterminals have been shown in the circuit of Fig. 3 for the reason that the resonance characteristics of each tunable circuit affect the other so that input and output terminals may be located at various selected points in the network.

In considering the operation of the circuit of Fig. 3, it will be seen that the characteristics and operation thereof are generally similar to those of the circuit of Fig. 1. Equation '4 indicates that the circuit of Fig. 3 is one in which the coefilcient of coupling is inversely proportional to the mean frequency of the pass band of the selector which is the correct relation for uniform band width; that is, the width of the pass band is dependent only on the values of the fixed reactance elements Ce, C2, Ll. Therefore, the desired uniform band width is obtained while tuning both the capacitance-tunable circuit and the inductance-tunable circuit to any selected frequency within the tuning range. While it is desirable that the capacitance-tunable circuit and the inductance-tunable circuit be tuned substantially to the same frequency, it will be understood that the unicontrol arrangement indicated in the drawing may be omitted.

The circuit of Fig. 4 may be derived from the circuit of Fig. 1 by another impedance transformation, leading to a fixed inductance coupling the band-width is much less than the mean frequency:

't o z/ o i ML r 1 L Aw=kw =rc (8) where,

Be is the susceptance of inductance Ll. B1 is the susceptance of one arm of the capacitance-tunable circuit, and B2 is the susceptance of one arm of the inductance-tunable circuit.

Incidental inductance in series with the coupling capacitor Cc of Fig. 3 may have the undeslr-- able effect of causing variation of the band width of the selector circuit as its frequency is adjusted over the tuning range. A circuit for compensating for this undesirable effect is shown in Fig. 5. The circuit of Fig. 5 is in all respects identical with the circuit of Fig. 3 except that the incidental inductance in series with the coupling capacitance Co is indicated as Lo and a compensating capacitance Cc" is added. The following equations applicable to the circuit of Fig. 5, are derived upon the premise that the net change of coupling due to the addition of elements Le and Cc" is zero: g

where k is the net change in coefiicient or coupling. Therefore,

Incidental capacitance across the coupling inductance Le 01' Fig. 4 may have a similar detrimental effect on the band width of the selector of Fig. 4., A circuit for compensating for the eflect of this incidental capacitance is shown in the circuit of Fig. 6. The circuit of Fig. 6 is identical with that ofFig. 4 with the exception that the incidental capacitance across inductance Le is represented as Ce and a compensating inductance Le" is included in a common portion of the two tunable circuits; that ,is, in series with each of the tunable circuits. The following equations for compensation apply to the circuit of Fig. 6: V

'C'JL, L -T,- (12) Incidental capacitance, across the fixed inductance ot the circuit of Fig. 3 may also have. a

similar detrimental effect on the band-pass charother terminals of these elements. The condition for compensation is the condition for balancing the bridge whose tour arms are the elements in this relation:

e Cl Therefore,

=C :IC2 C Inherent capacitance across the variable inductance. L: of Fig. 3 may also have a similar detrimental efi'ect upon the band width of the circuit. A circuit for compensating for this incidental capacitance is shown in Fig. 8. .The circuit of Fig. 8 is identical with that of Fig. 3 -except that the incidental capacitance across variable inductance L2 is represented by Cc, and compensating capacitance Cc" is connected in series with the coupling capacitance Cc, although the two elements Ce and Ca" may be embodied in a single physical element. The following equations for compensation are applicable to the cir-' cult of Fig. 8:

where, k'/k is-the ratio of the change in coupling k due to the addition of elements Cc and Ce" to the coupling k without these elements.

Therefore,

It will be understood that other incidental coupling efiects may exist in the circuits of Fig. 3 or Fig. 4 and that any or all of these may'be compensated by an extension of the principles illustrated in the compensating circuits of Figs. 5-8, inclusive.

In Fig. 9 there is shown a tunable selector circuit in accordance with the invention utilized as ments connected in cascade in the order named:

An intermediate-frequency amplifier H, a detector IS, a video-frequency amplifier l6, and an image-reproducing device i1, which may be a cathode-ray signal-reproducing tube. It will be understood that suitable scanning apparatus for the image-reproducing device I! and synchronizing apparatus therefor is included in the unit l6. The portions of the receiver described above may all be of conventional well-known construction so that detailed illustrations and dc:- scriptions thereof are deemed to be unnecessary.

Referring briefly, however, to the operation of the system described above, television signals intercepted by antenna II are selected by selector 9 and supplied to modulator ll wherein they are converted to intermediate-frequency signals which in turn are selectively amplified in intermediate-frequency amplifier I4 and delivered to detector IS. The modulation components of the signal are derived by the detector lland are Neglecting for the mo-,

supplied to the video-frequency amplifier I 6 wherein they are amplified and from which they are supplied in the usual manner to a brilliancycontrol element of the image-reproducing device IT. The intensity of the scanning beam of device H is thus modulated or controlled in accordance with the light-modulation voltages impressed on the control element of the device H in the usual manner. Synchronized scanning fields are generated and applied to the device ll, thereby to deflect the scanning ray in directions normal to each other so as to trace a rectilinear scanning pattern on the screen of the tube and reconstruct the transmitted image.

Referring now more particularly to the portion of the system of Fig. 9 embodying the present in- I vention, there is shown a fixed-inductance transcuit of the general type of Figs. 3 and'5 and,

utilizing inductances and 26 connected in parallel as the fixed inductance element of the capacitance-tunable first circuit thereof, there are provided condensers 21, 28, 29 which may be selectively connected into the tuned circuit by means of a switch 30. Theinductance-tunable second circuit of the selector comprises an inductance 3i, together with inductances 32 and 33 which may be selectively included in series with inductance 3| by means of a switch 3'3, to provide three values of inductance required for the three frequency bands to which the receiver of Fig. 9 is tunable. The tunable circuit including inductance 3i is'tuned by capacitance 36, shown in dotted lines, for the reason that it may be comprised in whole or in part of the inputelectrode capacitance of vacuum tube Ii. A fixed coupling condenser 31, shunted by grid leak 38, is effective to couple the two tunable circuits of the selector. Condenser 40, shown in dotted lines, to represent, in part or in whole, inherent circuit capacitance, serves to compensate for inherent inductance which may be present in series with coupling condenser 3'8.

Oscillator l2 comprises a tuning switch M ganged for unicontrol with switches and 34. It will be understood that switch 4| is effective to insert various tuning elements in the oscillator frequency-determining circuit to provide the required intermediate frequency for any desired received signal.

The design relations in Fig. 9 are chosen with reference to the band-pass selector to secure the damping required to give the same shape of resonance curve for any pass band to which the selector may be tuned. The requirement is uniform effective series resistance in the capacitance-tunable first circuit and uniform effective shunt conductance in the inductance-tunable second circuit.

In the application of the selector of Fig. 9, it is desired to secure maximum voltage ratio from the antenna in to the grid of tube l l. The resistance of the antenna and the line 2d cause the line to present uniform resistance in the primary circuit 20, 2|, 22, 23. This primary circuit is designed to couple uniform resistance effectively in series with the secondary or capacitance-tunable first circuit of the selector comprising inductances 25 and 26 in parallel. In order to secure this uniform resistance, the primary circuit is self-resonant at a frequency near the lower limit of the tuning range and has such small reactance that it is about half-critically damped by the line resistance.

The amount of uniform resistance coupled into the capacitance-tunable first circuit of the selector is determined by the-mutual inductance between primary and secondary circuits, measured with primary inductances 20, 28 in series and secondary inductances 25, 25 in parallel. This amount of resistance is preferably adjusted to secure sumcient damping of the first tunable circuit of the selector so that no damping of the second tunable circuit is required. This is the condition for maximum voltage ratio from the antenna III ,to the grid of modulator tube Ii. However, if there is any damping of the second tunable circuit, it should be by uniform effective shunt conductance in order to assure uniform effect on the shape of the resonance curve. The reason for unusual arrangement of the transformer 20, 2|, 25, 26 is to secure a symmetrical coil assembly for coupling the balanced primary circuit with the unbalanced secondary circuit. Such a transformer is described by applicant in his paper entitled Measurement of vacuum-tube capacities by a transformer balance}! Proceedings I. R. E., April, 1928, p. 476..

Inasmuch as the inductance-tunable second circuit of selector 9 requires constant capacitance and constant shunt conductance, which may be zero, it is important that the input conductance of vacuum tube ll be constant, preferably zero, over the frequency range. For this purpose, there is provided an inductance 42 in the" screen circuit of the tube II which, together with the inherent capacitance between the screen and the control electrode of tube ll, indicated in dotted lines in the drawing as capacitance 44, completes a feedback circuit to maintain the effective input conductance of tube H substantially zero. Since the grid conductance of vacum tube II, which is principally that due to the transit time of electrons, is proportional to the square of the frequency, it is desirable to proportion the feedback conductance g: in accordance with the following expression:

"g 0 0)) "3020M," where C33 is the grid-screen capacitance 44 and Ls is the screen inductance 42. Since both the input conductance and the negative feedback conductance are proportional to the square of the frequency, it is possible to make them compensate each other over the entire frequency range. This p: esent disclosure is disclosed and claimed in applicants divisional application, Serial No. 277,862,

input conductance compensating feature of the filed June 7, 1939, and assigned to'the same assignee as the present application.

From the foregoing, it will be clear that, as the switches 30, 34, and 4| are operated to adjust the tuning of the selector to any selected one of the three frequency bands to which the system is tunable, the band-pass characteristic is maintained uniform in width and of a constant shape.

A different arrangement for maintaining a subwhere M is the mutual inductance between inductances 3| and 45;

L2 ,is the total inductance of the inductancetunable circuit of the selector; and

C2 is the total capacitance of the inductancetunable circuit of the selector, namely that of condenser 36.

In Fig. 10, as in Fig. 9, the input conductance and the negative feedback conductance are proportional in magnitude so that they compensate each other over the frequency range.

While there have been described what are at present considered to be the preferred embodiments of the present invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is, therefore.

aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention,

What is claimed is: f

1. A band-pass selector tunable over a wide range of frequencies with uniform band width comprising, a capacitance-tunable circuit, an inductance-tunable circuit, and a fixed reactance element coupling said circuits, said reactance element being of such type with respect to its manner of coupling between said circuits that the resulting coefiicient of coupling is inversely proportional to frequency, whereby said bandwidth is maintained uniform as said circuits are tuned simultaneously to any frequency within said range.

2. A band-pass selector tunable over a wide range of frequencies'with uniform band width comprising, a capacitance-tunable circuit, an inductance-tunable circuit, and a fixed capacitance coupling said circuits, the manner of coupling of said fixed capacitance between said circuits being so chosen that the resulting coefiicient of coupling is inversely proportional tothe frequency, whereby said band width is maintained uniform as said circuits are tuned simultaneously to any frequency within said range.

3. A band-pass selector tunable over a wide range of frequencies with uniform band width comprising, a capacitance-tunable circuit, an inductance-tunable circuit, and a fixed capacitance common to said tunable circuits, the-mannerof coupling of said fixed capacitance between said circuits being so chosen that the resulting coemcient of coupling is inversely proportional to frequency, whereby said band width ismaintained uniform as said circuits are timed simultaneously to any frequency within said range.

4. A band-pass selector tunable over a wide range of frequencies with uniform band width comprising, a capacitance-tunable circuit, an in.-

ductance-tunable circuit, and, a fixed inductance said fixed inductance between said circuits being so chosen that the resulting coefiicient of coupling is inversely proportional to the frequency, whereby said band width is maintained uniform coupling said circuits, the manner of coupling of as said circuits are tuned simultaneously to any frequency within said range.

5. A band-pass selector tunable over a wide range of frequencies with uniform band width comprising, a capacitance-tunable circuit, an inductance-tunable circuit, and a fixed inductance coupling element, said circuits being coupled in parallel through said inductance coupling element, whereby said band width is maintained uniform as said circuits are tuned simultaneousl to any frequency within said range.

6. A band-pass selector tunable over a wide range of frequencies with uniform band width comprising, a first resonant circuit including in series an adjustable'capacitance, a fixed capacitance and a fixed inductance, and a second reso-- nant circuit including in series said fixed capacitance, another fixed capacitance and an adjustable inductance, whereby said band .width is maintained uniform as said circuits are tuned simultaneously to any frequency within said range.

'7. A band-pass'selector tunable over a wide range of frequencies with uniform band width comprising, a capacitance-tunable circuit including an adjustable capacitance and a fixed inductance in parallel, aninductance-tunable circuit comprising a fixed capacitance and an adrange of frequencies with uniform band width 7 comprising, a capacitance-tunable circuit, an inductance-tunable circuit, and a fixed reactance element coupling said circuits, unicontrol means for simultaneously adjusting the tuning elements of said circuits to tune them to the same frequency, the kind of reactance of said coupling element being so chosen with respect to its manner of coupling between said circuits that the resulting coefficient of coupling is inversely proportional to the frequency, whereby said band width is maintained uniform as said circuits are tuned simultaneously to any frequency within said range.

9. A band-pass selector tunable over a wide range of frequencies with uniform band width comprising, a capacitance-tunable circuit, an inductance-tunable circuit, and a fixed reactance element coupling said circuits, the reactance of said element being of such type with respect to its manner of coupling between said circuits that the resulting coemcient of coupling is inversely proportional to the frequency, said selector comprising incidental reactance tending to disturb the uniformity of said band width as said selector is tuned over its range, and means for compensating the effect of said incidental reactance on the width of the pus band of said selector, whereby said band width is maintained uniform as said circuits are tuned simultaneously to any frequency within said range.

10. A band-pass selector tunable over a wide range of frequencies with uniform band width comprising, a first resonant circuit including in series an adjustable capacitance, a fixed capacitance and a fixed inductance, and a second resonant circuitincluding in series said fixed capacitance, anuher fixed capacitance and an adjustable inductance, incidental inductance in series with said first-mentioned fixed capacitance tendingto disturb the uniformity of said band width as said selector is tuned over its frequency range, said adjustable capacitance and said other fixed capacitance having a common terminal, and means comprising an additional capacitance coupled between the other terminals of said adjustable capacitance and said other fixed capacitance to compensate for the effect of said incidental inductance on said band width, whereby said band width is maintained'uniform as said circuits are tuned simultaneously to any frequency within said range.

11. A band-pass selector tunable over a wide range of frequencies with uniform band width comprising, a first resonant circuit including in series an adjustable capacitance, a fixed capacitance and a fixed inductance, and a second resonant circuit including in series said fixed capacitance, another fixed capacitance and an adjustable inductance, incidental capacitance across said fixed inductance tending to disturb the uniformity of said band width as said selector is tuned over its frequency range, said adjustable capacitance and said other fixed capacitance having a common terminal, and an additional capacitance connected between the other terminals of said last-mentioned capacitances to compensate for the eifect of said incidental capacitance on said band width, whereby said band width is maintained uniform as said circuits are tuned simultaneously to any frequency within said range.

12. A band-pass selector tunable over a wide range of frequencies with uniform band width comprising, a resonant circuit tunable primarily by adjustable capacitance, a resonant circuit tunable primarily by adjustable inductance, and a coupling between said circuits comprisingprimarily a single reactance element, said reactance element being of such type with respect to its manner of coupling between said circuits that the resultant coeificient of coupling between said circuits is inversely proportional to frequency, whereby said band width is maintained uniform as said circuits are tuned simultaneously to any fre quency within said range.

' 'i A. WHEELER. 

